Prepare the Data

Please note that the two tasks in this section (cifbuild and
odfingest) must be run in the ODF directory. These are the
only tasks with that requirement, and after this section, we will
work exclusively in our reprocessing directory.

Many SAS tasks require calibration information from the Calibration
Access Layer (CAL). Relevant files are accessed from the set of Current
Calibration File (CCF) data using a CCF Index File (CIF). To make the
ccf.cif file, navigate into the ODF directory and in the Command Window, type

cifbuild

The task odfingest extends the Observation Data File (ODF) summary
file with data extracted from the instrument housekeeping data files and
the calibration database. It is only necessary to run it once on any
dataset, and will cause problems if it is run a second time. If for some
reason odfingest must be rerun, you must first delete the earlier
file it produced. This file largely follows the
standard XMM naming convention, but has SUM.SAS
appended to it.
To run odfingest, just type in the Command Window

odfingest

Hera automatically resets the relevant environmental parameters to the
output of these tasks, so we can continue merrily on our way.

Reprocess the Data

To reprocess the data, go up one directory in the tree and make a new working
directory using the buttons in the upper left corner of the User Account Window.
When you are in the working directory, call either emproc or
emchain. In the Command Window, type

emproc

or

emchain

By default, these tasks do not keep any intermediate files they generate.
Emchain maintains the
usual naming convention. Emproc designates
its output event files with "TimingEvts.ds".
In any case, it is convenient to rename them something easy to type;
this can be done by clicking on the pen icon next to the file name in the
User Account Window. We'll assume the new name for the event file is
mos1_te.fits.

If you are likely to want to extract a background spectrum
for your source, you will also need to consider the imaging
event file. We might as well deal with that while we're here.
Remember that whatever filtering is done on the timing
event file must also be done on the image event file.
We will rename the image event file mos1_ie.fits.

Make a Light Curve

The XMM-Newton Observatory is susceptible to soft particle flaring, so it is
necessary to examine the light curve to determine how much of the data is
useful.

table - input event table rateset - name of output light curve filemaketimecolumn - make a time columntimebinsize - time binning (seconds)makeratecolumn - make a count rate column, otherwise a count column will be created

The output file mos1_ltcrv.fits can be viewed by downloading it and
using fv, as shown in Figure 1.

fv mos1_ltcrv.fits &

Figure 1:
The Mkn 841 light curve.

Apply Standard Filters

The filtering expression for the MOS in TIMING mode is:

(PATTERN <= 12)&&(PI in [200:12000])&&#XMMEA_EM

The first two expressions will select good events with PATTERN in the 0 to 12 range. The
PATTERN value is similar the GRADE selection for ASCA data, and is related to the number
and pattern of the CCD pixels triggered for a given event. Single pixel events have
PATTERN == 0, while double pixel events have PATTERN in [1:4] and
triple and quadruple events have PATTERN in [5:12].

The second keyword in the expressions, PI, selects the preferred pulse height of the
event. For the MOS, it should be between 200 and 12000 eV. This should clean up the
image significantly with most of the rest of the obvious contamination due to low pulse
height events. Setting the lower PI channel limit somewhat higher (e.g., to 300 or 400 eV)
will eliminate much of the rest.

Finally, the #XMMEA_EM filter provides a canned screening set
of FLAG values for the event. (The FLAG value provides a bit encoding of various event
conditions, e.g., near hot pixels or outside of the field of view. Setting
FLAG == 0 in the selection expression provides the most conservative
screening criteria and usually is not necessary for the MOS.)

Apply Time Filters

Sometimes, soft proton background flaring makes it necessary to use filters
on time in addition to those mentioned above.

To determine if there is flaring in the observation,
make a light curve and display it.
No flares are evident, so we will continue to the next section. However, if
a given dataset does contain flaring, it should be removed in the same way
as shown for EPIC IMAGING mode data
here.

If needed, we can also extract a background spectrum. For this, we will use
the imaging event list, since we want the background to be as far away from
the source as possible. As with the source spectrum, we will need to make
an image first.

Figure 3:
The filtered imaging event file for Mkn 841. The green box indicates
the region where the background spectrum was extracted.

Determine the Spectrum Extraction Areas

The source and background region areas can now be found. (This process is identical to that
used for IMAGING data.) This is done with the task backscale, which takes into account any
bad pixels or chip gaps, and writes the result into the BACKSCAL
keyword of the spectrum table. To find the source and background extraction areas:

Check for Pile Up

Depending on how bright the source is and what modes the EPIC detectors are in, event pile
up may be a problem. Pile up occurs when a source is so bright that incoming X-rays strike
two neighboring pixels or the same pixel in the CCD more than once in a read-out cycle. In
such cases the energies of the two events are in effect added together to form one event.
Pile up and how to deal with it are discussed at length
here
and
here,
respectively, and users are strongly encouraged to refer to those sections. Briefly, we deal
with it in MOS TIMING data essentially the same way as in IMAGING data, that is, by using only
single pixel events, and/or removing the regions with very high count rates, checking the
amount of pile up, and repeating until it is no longer a problem.
In our example case, the source is far too faint to be piled up, but it is always a good
idea to check anyway.
To check for pile up:

Making the RMF for MOS data in TIMING mode is exactly the same as in IMAGING mode, which is
demonstrated
here. If the boresight was not excised (as may be necessary if your data is piled up),
making the ARF is the same, as well.

However, if you had to remove piled up sections, you will need to make an ARF for the full extraction
area, another one for the piled up area, and then subtract the two to find the ARF for the non-piled regions.
Users are referred to
this discussion,
where the procedure is demonstrated on PN TIMING data.

The spectrum can be fit using HEASoft or CIAO packages, as SAS does not include fitting software.